All tadpoles turn into frogs, but not all frogs are turned from tadpoles. Half of the 4,800 known frog species start life as eggs laid in water, hatch into aquatic tadpoles, then metamorphize into frogs. The other half do something else. Frog life cycles are incredibly diverse.
Start with the egg. Some are planted on leaves, others in foam nests, with the foam fabricated by mom before laying the eggs. A female may instead carry her eggs: on her back, or in her throat or stomach.
Hundreds of species skip the tadpole stage, directly developing from eggs to tiny toads that grow into warty adults. Such frogs invariably have exotrophic larvae: little ones who get their nutrition from the environment. In contrast, tadpoles always start out in eggs with a built-in food supply (endotrophic larvae).
The diversity of development reflects life-history variables: aspects of the many choices that evolution provides for adaptation to a habitat. Life-history variables are always related. With regard to frogs, direct development coincides with: wetter and warmer regions, where eggs do not desiccate if not laid in water; reduced number of eggs laid; and smaller adult size.
All genomes are a library of life-history options that may be selected based upon environmental conditions. If an option does not exist in the library, its evolution is had by configuration from previously known genetic elements, along with a coherent spark of adaptive creativity.
Ivan Gomez-Mestre, et al, “Phylogenetic analyses reveal unexpected patterns in the evolution of reproductive modes in frogs,” Evolution (15 July 2012).